Optical absorption measurements have long been used to measure the concentration of a component in a mixture. The absorption behaviour can be described by the Beer-Lambert law. A simple form of an absorption spectrometer consists of a light source, a means to select the relevant wavelength range, a sample chamber and detector. The reduction in the transmitted light intensity when the absorbing component is present allows the concentration of the component to be deduced. Whilst this may give sensitive readings for the component of interest, depending on the absorption strength and path length, it may be subject to zero and sensitivity errors due to changes in the source intensity and cross interference (measurement error) if other components are present which also absorb within the wavelength pass band.
Zero and sensitivity errors can occur because there is no reference to the change in intensity of the source with time. This effect can be compensated for by using a reference measurement which monitors the source output simultaneously with the measurement of interest. The cross interference can be minimised by using a selective detection system, tuned to the specific absorption of the component of interest.
Another approach is by using a single optical path gas filter correlation measurement. The basic lay-out of which is illustrated in FIG. 1. In this method, light from a broad band source 1 is mechanically modulated using a wheel 2 containing a cuvette 3 filled with non-significantly absorbing gas, such as nitrogen and another cuvette 4 containing the gas of interest. In order to increase the signal to noise ratio, an optical band pass filter 5 is used to select a region of interest. The resultant modulated emission is directed into a measurement cell 7, where further absorption may occur, depending on the sample composition. The signal is collected at an optical detector 8 where it is converted into an electrical signal, which is then processed by processing circuitry 9 to produce an external output signal.
The signal from the optical detector will consist of modulated output corresponding to the throughput from the different gas filled cuvettes. The following will describe the simplified case of a single, non-absorbing cuvette filled with nitrogen and a single, absorbing cuvette filled with the gas of interest, in this case, NO (nitric oxide). Although this simple illustration is given, this description applies equally well to any material being measured by any spectroscopic technique such as using absorption or reflection.
Returning to our example, a schematic of the output signals is illustrated in FIGS. 2a and b for the case of non-absorbing mixture (nitrogen) in the sample cell. The signal which has gone through the NO cuvette will always have a smaller amplitude than the signal through the nitrogen cuvette, since some of the light corresponding to the absorption spectrum has been absorbed. Different gains are applied to the two signals, by the processing circuitry 9 such that the magnitudes of the two signals are matched. This is taken as the zero point in deriving the concentration measurements.
Thereafter, when a sample containing the gas of interest is introduced to the sample cell, for example NO in a nitrogen background, a change in the output signal occurs (FIGS. 2c and d). The change in the nitrogen cuvette signal is always larger than that of the NO cuvette, since the NO cuvette has already pre-absorbed a portion of the radiation corresponding to the NO absorption bands. The amount of incident radiation absorbed by the NO cuvette will depend on, for example, its concentration, temperature, background gas and absorption path length. The difference between the two gained signals, ΔI in FIG. 2d, is related to the NO concentration in the sample gas cell and so this sets the sensitivity for the measurement. The difference is normally divided by the NO cuvette signal to give a normalised signal, which is independent of any changes in the source intensity and then multiplied by an instrumentation factor to give an NO concentration reading.
In the preceding example, the effect of background interferent gas(es) can cause an error in the reading if they absorb within the pass band of the filter. The example described above provides for two types of interference to occur: positive interference, which causes a positive error, where the interferent absorption coincides with an NO absorption bands and negative interference, which causes a negative error, where the absorption does not coincide with an absorption band. This is illustrated in FIG. 3. Positive interference results from an inherent inability to distinguish between NO and the interferent, but negative cross interference results from the normalised differential gain applied to the signals (i.e. proportional to (G-1), where G is the normalised differential gain applied). Often both types of interference can occur simultaneously.
The magnitude of the cross interference for a particular gas mixture can be minimised through choice of the pass band of the optical filter and characteristics of the gas cuvettes. However, in some cases, such as with water as an interferent gas for NO, it may not possible to remove the interference satisfactorily, resulting in unacceptable error.
A common method of dealing with the effect of cross interference is to independently measure the concentration of the cross interferer, whether by gas filter correlation or other means, and correct accordingly. This, of course, requires extra equipment for a second measurement together with increased cost and complexity. Additionally, the sample would not be identical to that seen by the primary measurement, but could have temporal and/or spatial separation.